In the past, there has been known an organic EL element which includes a stack in which an organic light-emitting layer is between a pair of electrodes. In this organic EL element, the organic light-emitting layer produces light when electrons and holes are recombined inside the organic light-emitting layer. Further, there has been known another organic EL element having a multi-unit structure which includes multiple light-emitting units (for example, see Patent Literature 1 to 3).
Patent literature 1 relates to an organic EL element in which multiple light-emitting units are connected in series, and discloses a method for obtaining light with desired chromaticity by mixing light from the multiple light-emitting units. In this organic EL element, to realize the white light emission, two or more fluorescent light-emitting units are provided with regard to one phosphorescent light-emitting unit. With this method, although the desired chromaticity can be easily realized, the total luminous efficiency tends to decrease because the fluorescent light-emitting units whose luminous efficiency is lower than that of the phosphorescent light-emitting unit are stacked. Thus, there has been a need for improvement in terms of the luminous efficiency.
Moreover, patent literature 2 discloses a method for obtaining an organic light-emitting element with a high luminous efficiency by stacking a multi-color light-emitting unit and a monochromatic light-emitting unit whose luminous efficiency is lower than that of the multi-color light-emitting unit. Specifically, to realize high luminous efficiency, a stacked structure of a fluorescent red light-emitting unit and a fluorescent blue-green light-emitting unit, or a stacked structure of a fluorescent blue light-emitting unit and a phosphorescent green-red light-emitting unit is used. However, in consideration of lighting use, the former is a double layer structure of fluorescent light-emitting units, and is lower in the luminous efficiency than a structure including phosphorescent light-emitting units. The latter is a structure of which a green light-emitting layer and a red light-emitting layer are stacked in the unit, and a driving voltage of the unit is likely to be affected by green light emission corresponding to a large band gap. Therefore, compared with a red monochromatic light-emitting unit, the driving voltage is relatively high, and as a result the luminous efficiency becomes low.
Patent literature 3 discloses a structure for suppressing change with time in color by adopting a structure in which a first light-emitting unit including multiple organic light-emitting layers and a second light-emitting unit including an organic light-emitting layer with a single layer structure are stacked. As a white element including a two-unit structure, (1) a stack of a fluorescent light-emitting unit and a fluorescent light-emitting unit, (2) a stack of the fluorescent light-emitting unit and a phosphorescent light-emitting unit, (3) a stack of the phosphorescent light-emitting unit and the phosphorescent light-emitting unit, and the like are considered. However, the aforementioned (1) is the lowest in the luminous efficiency. The aforementioned (3) requires a blue phosphorescent material and therefore it may leave a big problem on a lifetime. The aforementioned (2) is a structure which is good in both the luminous efficiency and the lifetime, but it also may leave a problem on the luminous efficiency because the fluorescent light-emitting units and the phosphorescent light-emitting units are stacked at the number ratio of 1:1 and an advantageous feature of the phosphorescent light-emitting unit with high luminous efficiency cannot be sufficiently utilized.